Backlight module for a liquid crystal display

ABSTRACT

A backlight module for an LCD includes a light guide plate having a light incident surface and a light output surface adapted to face a display panel. A light source is provided for emitting light beams to the display panel from the light incident surface via the light output surface. And, it also includes a set of substantially identical bead-structured sheets stacked together and placed on the light output surface of the light guide plate for uniformly enhancing the light beams emitted from the light source without using any additional brightness enhancement sheet.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a backlight module for a Liquid CrystalDisplay, more specifically to a backlight module for an LCD that uses aset of substantially identical bead-structured sheets for brightnessenhancement without using additional brightness enhancement sheets.

2. Description of the Related Art

It is known that the LCD uses a backlight module for illumination.Referring to FIG. 1, a conventional backlight module 10 includes a lightguide plate 11, a light source 12, a reflector plate 13 and a set ofoptical sheets 14.

The light source 12, preferably a cold cathode fluorescent lamp, ispositioned at edgewise manner relative to the light guide plate 11. Asillustrated in FIG. 1, for a single lamp design, the light guide plate11 is usually wedge-shaped for providing uniform illumination. The lightguide plate 11 is generally made from transparent acrylic resin. It hasa light incident surface 112 and a light output surface 111. The lightguide plate 11 is used for changing the point light source or the linearlight source into the plane light source. The bottom surface of thelight guide plate 11 is formed with a reflection pattern 113 for guidingthe light from the light incident surface 112 towards the light outputsurface 111. The reflector plate 13 is positioned below the light guideplate 11 for reflecting the light beams towards the light output surface111 to enhance illumination.

The set of optical sheets 14 is stacked together and positioned on thelight output surface 111 of the light guide plate 11. The arrangement ofthe set of optical sheets 14 depends on application. The set of opticalsheets 14 typically includes a bottom light diffusing sheet 141, a prismsheet 142, and a top light diffusing sheet 143 for products of centerbrightness request. The top and bottom light diffusing sheets 141, 143cooperatively provide uniform light diffusion while the prism sheet 142provides brightness enhancement. For high brightness request, the set ofoptical sheets may include a bottom diffuser, a BEF III (trade name ofthe 3M Company), and a DEBFD (trade name of the 3M Company) from bottomto top arranged as a set.

Prism sheets available in the commercial market include BrightnessEnhancement Film (BEF, trade name of a product by the 3M Company) andDual Brightness Enhancement Film (DBEF, trade name of a product by the3M Company). It is noticed that the prism sheet is relatively expensivein comparison with the cost of light diffusing sheets. For example, thecosts of the BEF III and the DBEF by 3M are respectively 5 times and 10times the cost of a light diffusing sheet.

As illustrated in FIG. 2, the prism sheet 142 a by 3M is made fromtransparent plastic, such as PMMA (polymethyl-methacrylate), and isformed with arrays of prisms 15 on the surface for recollection of lightbeams reflected or refracted while traveling through a light guideplate.

In addition to the high cost, another disadvantage of using a prismsheet, such as BEF III or DBEF, is that it must be used together with atleast a top and a bottom light diffusing sheet for providing uniformlight diffusion and avoiding scratches during handling the assembly. Asa result, it is inevitable to increase additional cost.

SUMMARY OF THE INVENTION

In view of the foregoing disadvantages of using the conventional opticalsheets for brightness enhancement and light diffusion, an object of thepresent invention is to provide a backlight module with cost-reducedoptical sheets capable of brightness enhancement and light diffusion.

According to the present invention, the backlight module includes alight guide plate having a light incident surface and a light outputsurface adapted to face a display panel. A light source is provided foremitting light beams to the display panel from the light incidentsurface via the light output surface. And, it also includes a set ofsubstantially identical bead-structured sheets stacked together andplaced on the light output surface of the light guide plate foruniformly enhancing the light beams emitted from the light source. Theset of bead-structured sheets can achieve desired optical effectswithout having to use any additional brightness enhancement sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a backlight module showing thestructure of a backlight module for a conventional Liquid CrystalDisplay;

FIG. 2 is a schematic diagram showing the cross-sectional view of theconventional optical sheets in which BEF III is employed;

FIG. 3 is a schematic diagram showing the cross-sectional view of apreferred embodiment of the present invention;

FIG. 4A is a schematic diagram showing another preferred embodiment ofthe present invention;

FIG. 4B is a schematic diagram showing still another preferredembodiment of the present invention;

FIG. 5 is a schematic diagram illustrating light transmission paths in abead-structured sheet according to the present invention;

FIG. 6A is a plan view showing the measurement positions on the panelwhen conducting a brightness measurement;

FIG. 6B is a graph showing the relationship between the number ofbead-structured sheets stacked together and associate brightnessmeasured at the measurement positions as shown in FIG. 6A;

FIG. 7A is a graph showing the change of brightness at differenthorizontal viewing angles when using Light Guide Plate only, threebead-structured sheets, and BEFIII respectively;

FIG. 7B is a graph showing the change of brightness at differentvertical viewing angles when using Light Guide Plate only, threebead-structured sheets, and BEFIII respectively and

FIG. 8 is a table showing the relationship between the number ofbead-structured sheets stacked together and correspondent brightnessmeasured at different perpendicular viewing angles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 3, the backlight module 30 includes a light guideplate 31, a light source 32, and a set of bead-structured sheets 33according to one of the preferred embodiments of the present invention.The light source 32, preferably a cold cathode fluorescent lamp, ispositioned at edgewise manner relative to the light guide plate 31 forproviding side light.

The light guide plate 31, usually made from transparent acrylic resin,has a light incident surface 312 and a light output surface 311, whichis positioned to face the display panel 20. The light guide plate 31changes the linear light source 32 into the plane light source andguides the light beams towards the display panel 20.

The lower surface of the light guide plate 31 is formed with a pluralityof transparent beads 313 for light diffusion. A reflector plate 34 ispositioned below the light guide plate 31 for enhancing brightness byreflecting reflected or refracted light beams towards the display panel20.

The bead-structured sheets 33 are substantially identical. They arestacked together and positioned on the light output surface 311 of thelight guide plate 31. When light beams are guided to travel through thebead-structured sheets 33 towards the display panel 20, they will bemore uniformly diffused but still converge enough light to achieveuniform illumination and brightness enhancement without using anyconventional brightness enhancement sheets, such as prism sheets.

The set of bead-structured sheets 33 is preferably formed by stackingthree substantially identical bead-structure sheets together. Accordingto the experiment results, the luminance angle of triple bead-structuredsheets is smoother than that of the conventional BEF III. As a result,light beams will be more uniformly distributed all over the displaypanel 20. Accordingly, the set of bead-structured sheets can comply withthe safety requirement of TFT TCO'03.

FIG. 4A shows another preferred embodiment of the backlight module ofthe present invention which has the structure similar to the embodimentof FIG. 3. The main difference is that the light guide plate 31 a is ofsubstantially uniform thickness. It has two light incident surfaces 312a. Two cold cathode fluorescent lamps 32 a are positioned at edgewisemanner respectively adjacent to the light incident surfaces 312 a of thelight guide plate 31 a. The remaining parts are the same.

FIG. 4B shows still another preferred embodiment of the presentinvention. Its structure is similar to that of the first preferredembodiment of FIG. 3 except the light source. The light incident surface312 b is positioned opposite to the light output surface 311 b of thelight guide plate 31 b due to the position of the direct light source 32b. The remaining parts are the same. The number of bead-structuredsheets used depends on actual applications.

FIG. 5 illustrates an enlarged view of the encircled portion (S) of thebead-structured sheets 33 as shown in FIG. 3. Each of thebead-structured sheets 33 is formed from a transparent base sheet 331. Asurface layer 332 is laminated on an upper surface of the base sheet331, and a sticking-inhibiting layer 333 laminated on the rear surfaceof the base sheet 331.

The base sheet 331 is formed from a transparent and colorless syntheticresin, including PET (polyethylene terephthalate), polyethylenenaphthalate, acrylic resin, polycarbonate, polystyrene, polyolefin,cellulose acetate, weather-resistant vinyl chloride and Polyester. Thesurface layer 332 includes a binder 334, preferably a thermosettingresin, and transparent beads 335 of different dimensions.

The transparent beads 335 are substantially spherical in shape. They canbe made from transparent and colorless material, such as acrylic resin,polyurethane, polyvinyl chloride, polystyrene, polyacrylonitrile andpolyamide. The size of the beads is preferably within the range of fromabout 0.1 μm to about 100 μm. The transparent beads 335 are scatteredrandomly within the binder 334 for light diffusion.

The sticking-inhibiting layer 333 is formed from a highly wear resistantbinder 334 to prevent from scratch. The sticking-inhibiting layer 333contains multiple transparent beads 335 scattered in the binder 334 toenhance sticking inhibition.

There are several bead-structured sheets available in the market for thepurpose of light diffusion rather than brightness enhancement. Forinstance, a bottom light diffusing sheet made by Keiwa under the modelname of Keiwa BS-040 is applicable to the present invention.

According to the preferred embodiment of the present invention, threebead-structured sheets as a set can provide desirable uniform brightnessenhancement effect without using any additional prism sheets or anyother brightness enhancement sheet. Moreover, it does not need anyadditional brightness enhancement sheet because the measured luminanceangle of the bead-structured sheets is smoother than that of theconventional BEF III.

FIG. 6A illustrates the measurement spots of the backlight module 30when conducting the brightness measurement. The measurement referencesand conditions are as follows: in the measurement, we use 7.5 mA coldcathode fluorescent lamp by TOA Co. as the light source, and a lampreflector of ALSET E60V. We use three bottom light diffusing sheets ofKeiwa BS-040 as a set of bead-structured sheets for brightnessenhancement.

Table 1 shows the relationship between the number of bead-structuredsheets used and associate brightness variation when measured at thepositions as shown in FIG. 6A. It shows that the maximum brightnessenhancement can be achieved when three bead-structured sheets of KeiwaBS-040 are stacked together as a set. Accordingly, for this arrangement,three sheets of Keiwa BS-040 are enough to achieve the desirablebrightness.

FIG. 6B shows the relationship between the number of bead-structuredsheets used in the backlight module and associate brightness variation.One can clearly observe that the brightness does not change much whenthe number of bead-structured sheets is increased from three to four.Based on the concern of cost, applying three pieces of bead-structuredsheets stacked together as a set can be the most cost-effectivesolution. However, the number of bead-structured sheets actually used isbased on application. Under the present experimental environment, threeKeiwa BS-040 sheets as a set can meet the center brightness requirementwithout using any additional prism sheet or any other brightnessenhancement sheet or any light diffusing sheet.

FIGS. 7A and 7B are graphs representing experiment results of brightnessmeasured at various horizontal and vertical viewing angles with respectto the center position of the backlight module of the present invention.From the experiment results, it can be shown that the light convergenceeffects are almost the same for using three bead-structured sheets as aset and using conventional BEF III. However, using three-bead structuredsheets as a set can achieve wider viewing angles than using conventionalBEF III.

Accordingly, the present invention can easily meet the safetyrequirement of TFT TCO'03. More importantly, using the threebead-structured sheets as a set will not generate moire as usingconventional optical sheets. And the bead-structured sheets areavailable in the market with a price lower than that of conventionalprism sheets. Furthermore, the bead-structured sheets with a highly wearresistant binder in the sticking-inhibiting layer can also avoidscratches during handling the assembly, thereby increasing the yieldrate.

As is understood by a person skilled in the art, the foregoing preferredembodiment of the present invention is an illustration of the presentinvention rather than limiting thereon. It is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the appended claims, the scope of which should be accorded thebroadest interpretation so as to encompass all such modifications andsimilar structure. TABLE 1 center Brightness 1 2 center 4 5 gain Lightguide 1769 1802 2099 1791 1817 — Bead-structured sheet × 1 3074 31453657 3155 3195 1.74 Bead-structured sheet × 2 4069 4149 4772 4159 41892.27 Bead-structured sheet × 3 4511 4581 5244 4601 4601 2.50Bead-structured sheet × 4 4611 4662 5295 4662 4652 2.52(nits)

1. A backlight module for a Liquid Crystal Display comprising: a lightguide plate having a light incident surface and a light output surfaceadapted to face a display panel; a set of substantially identicalbead-structured sheets stacked together and placed on said light outputsurface of said light guide plate; and a light source for emitting lightbeams to the display panel, the light beams traveling from said lightincident surface through said set of substantially identicalbead-structured sheets and directly to the display panel.
 2. Thebacklight module according to claim 1, wherein said light source ispositioned at an edgewise manner with respect to said light guide plate.3. The backlight module according to claim 1, wherein said light sourceis positioned opposite to the light output surface of said light guideplate.
 4. The backlight module according to claim 1, wherein each ofsaid set of substantially identical bead-structured sheets is formedfrom a base sheet, a surface layer laminated on an upper surface of saidbase sheet, and a sticking-inhibiting layer laminated on a rear surfaceof said base sheet.
 5. The backlight module according to claim 4,wherein said base sheet is formed from a transparent material selectedfrom a group consisting of polyethylene terephthalate, polyethylenenaphthalate, acrylic resin, polycarbonate, polystyrene, polyolefin,cellulose acetate, weather-resistant vinyl chloride and Polyester. 6.The backlight module according to claim 4, wherein said surface layerincludes a binder and a plurality of transparent beads of differentdimensions scattered randomly within said binder.
 7. The backlightmodule according to claim 4, wherein said sticking-inhibiting layerincludes a binder and a plurality of transparent beads scattered in saidbinder.
 8. The backlight module according to claim 1, wherein said setof substantially identical bead-structured sheets includes at least twosubstantially identical bead-structured sheets.